29th International Cosmic Ray Conference Pune (2005) 00, 101-104

Telescope Array; Status and Prospects
The Telescope Array (TA) Collaboration
H.Kawaia, T.Nunomuraa, N.Sakuraia, S.Yoshidaa, H.Yoshiib, K.Tanakac, F.Cohend,
M.Fukushimad, N.Hayashidad, M.Ohnishid, H.Ohokad, S.Ozawad, H.Sagawad, T.Shibatad,
H.Shimodairad, M.Takedad, A.Taketad, M.Takitad, H.Tokunod, R.Toriid, S.Udod, H.Fujiie,T.Matsudae, M.Tanakae, H.Yamaokae, K.Hibinof, T.Bennog, M.Chikawag, T.Nakamurah,
M.Teshimai, K.Kadotaj, Y.Uchihorik, Y.Hayashil, S.Kawakamil, K.Matsumotol,Y.Matsumotol, T.Matsuyamal, S.Ogiol, A.Ohshimal, T.Okudal, D.R.Bergmanm,G.Hughesm, S.Strattonm, G.B.Thomsonm, N.Inouen, Y.Wadan, K.Kasaharao, M.Fukudap,
T.Iguchip, F.Kakimotop, R.Minagawap, Y.Tamedap, Y.Tsunesadap, J.W.Belzqs,
J.A.J.Matthewsr, T.Abu-Zayyads, R.Cadys, Z.Caos, P.Huentemeyers, C.C.H.Juis,
K.Martenss, J.N.Matthewss, J.D.Smiths, P.Sokolskys, R.W.Springers, S.B.Thomass,
L.R.Wienckes, T.Doylet, M.J.Taylort, V.B.Wickwart, T.D.Wilkersont, K.Hashimotou,K.Hondau, T.Ishiiu, T.Kanbeu(a) Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba, 263-8522 Japan(b) Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan(c) Hiroshima City University, 3-4-1 Ozuka-Higashi, Asa-Minami-Ku, Hiroshima, 731-3194 Japan(d) ICRR, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8582 Japan(e) Institute of Particle and Nuclear Studies, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801 Japan(f) Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa, 221-8686 Japan(g) Kinki University, 3-4-1 Kowakae, Higashi-Osaka City, 577-8502 Japan(h) Kochi University, 2-5-1 Akebonocho, Kochi, 780-8520 Japan(i) Max-Planck-Institute for Physics, Foehringer Ring 6, 80805 Muenchen, Germany(j) Musashi Institute of Technology, 1-28-1 Tamazutsumi, Setagaya-ku, Tokyo, 158-8557 Japan(k) National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-shi, 263-8555 Japan(l) Osaka City University, 3-3-138 Sugimotocho, Sumiyoshi-ku, Osaka, 558-8585 Japan(m) Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA(n) Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama, 338-8570 Japan(o) Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama, 337-8570 Japan(p) Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8550 Japan(q) University of Montana , 32 Campus Drive, Missoula, MT 59812, USA(r) University of New Mexico, Albuquerque, NM 87131 USA(s) University of Utah, 115 S 1400 E, Salt Lake City, UT 84112, USA(t) Utah State University, Logan UT 84322, USA(u) Yamanashi University,4-4-37 Takeda, Kofu, Yamanashi, 400-8510 Japan

Presenter: M. Fukushima (fukushim@icrr.u-tokyo.ac.jp), jap-fukushima-M-abs1-HE15-oral
The Telescope Array (TA) is the air shower experiment designed for observing extremely high energy
cosmic rays. It consists of an array of scintillation counters covering the ground area of 760 km2 and 3 setsof air fluorescence telescopes placed in the perimeter of the array. The TA aims at making a decisive
measurement of the cosmic ray spectrum in the GZK cutoff region, and pinning down the nature of cosmic
rays exceeding 1020 eV, which have been the subject of extensive studies by AGASA, HiRes and other airshower experiments. The TA is being constructed in Utah, USA, and the commissioning is expected in thespring of 2007. An extension of hybrid measurement is planned down to 1017.5 eV in order to elucidate thegalactic to extra-galactic transition of cosmic ray sources (TALE proposal).

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1. Nature of Extremely High Energy Cosmic Rays (EHECRs)
The EHE proton propagating a long distance through the extragalactic space is expected to lose its energy byinteracting with the cosmic microwave background (CMB). Following the Lorentz invariance, the CMB isequivalent to a gamma ray of energy ~100 MeV in the rest frame of 1020 eV proton. Thus the proton of thisenergy is expected to result in a pion photo-production, and a significant part of its initial proton energy iscarried away by the produced pions. This was first predicted by Greisen, Zatsepin and Kuzmin (GZK) [1].
Two outcomes of the theory are;1.

The energy spectrum of EHECRs arriving from uniformly distributed sources in the universeshould exhibit a clear cutoff structure at ~1020 eV (= GZK cutoff).

2.

If the cosmic rays above 1020 eV (= super-GZK events) are detected, most of them should have anorigin within 50 Mpc of the Earth.

Despite the prediction of GZK, the energy spectrum measured by AGASA [2] shows no indication of theGZK cutoff [3]. The AGASA recorded 11 super-GZK cosmic rays in 13 years of operation. Moreover, in thesample of 59 events above 1019.6 eV, 13 events were found to form clusters of 2 or 3 events (5 doublets and 1triplet) indicating they originate from a common "point source" in the sky [4]. A correlation with highenergy astronomical objects such as active galactic nuclei, radio galaxy lobes and gamma ray bursts havebeen searched but no obvious candidates were identified within 100 Mpc [5]. In order to circumvent thedifficulties met with the astrophysical models, several particle physics oriented models have been proposed
for the origin of EHECRs. They are the decay of super-heavy relic particles in the galactic halo, theinteraction of EHE neutrinos with the cosmic neutrino background in the local cluster, the violation ofLorentz invariance at the extremely high γ factor and others [6].

2. Decisive Measurement of GZK Spectrum
The HiRes air fluorescence telescope, on the other hand, recently reported an energy spectrum which isconsistent with the existence of the GZK cut-off [7]. After floating the energy scale error of each experiment,~18% for AGASA and ~25% for HiRes, the spectra of both experiments agree well below 1020 eV, yet leavea 3σlevel of disagreement on the existence of GZK cutoff. It should be noted that two experiments differ
not only in the experimental method but also in the measured shower parameters; the AGASA measures thelateral shower profile in the shower periphery and the HiRes measures the longitudinal shower profile at theshower center. The discrepancy above 1020eV therefore may very well originate from the physics of theshower development at EHE or the nature of the incoming particles. The best way to resolve thiscontradiction is to measure the air shower simultaneously with an AGASA type air shower array and aHiRes type air fluorescence telescope.
The hybrid TA is proposed as the first step of building the full TA [8] with the urgent questions in mind;
whether the EHECR spectrum continues or ends at the GZK energy and whether the discrepancy between
AGASA and HiRes is from the statistics, the systematics or the physics. The configuration of hybrid TA isshown in Figure 1. The ground detector consists of an array of 576 plastic scintillators with an area of 3 m2
deployed in a grid of 1.2 km spacing covering the ground area of ~760 km2. The detector acceptance isapproximately 9 times that of AGASA. The detection efficiency is 100% for cosmic rays with energies morethan 1019.5 eV with zenith angles less than 450. The fluorescence measurement is made at 3 stations
surrounding the ground array. The stations form a triangle with a separation of 30 - 40 km. Twelve reflecting
telescopes are installed at each station and covers the sky of 30-340 in elevation and 1080 in azimuth looking

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toward the center of the ground array. The diameter of the telescope mirror is ~3.3 m and the pixel resolution

Figure 1. Detector Arrangement of Telescope Array. Three square boxes indicate the location offluorescence telescope stations overlooking a scintillation counter array at the center. The circles
indicate the locations of communication tower, by which all the controls and data acquisition arerelayed to the operation center in the nearby town of Delta.

is approximately 10 x 10. The stereo acceptance is ~670 km2 sr for E > 1020eV by requiring at least one
station is within 45km from the shower center. The fluorescence acceptance is 4 times that of AGASA
assuming 10% duty factor for observation. Details of the TA detector design and performance as well as theproposal for low energy extension (TALE) are reported elsewhere in this conference.

3. Status and Prospects
The ground array of hybrid TA uses the plastic scintillator, which is sensitive to the charged particles in theair shower. For energies greater than 1020eV, more than 90 % of the primary energy is transferred to theelectromagnetic component (e+, e － and γ) at the end of the shower development. Since electrons andpositrons outnumber muons by an order of magnitude, the thin (~12mm) plastic scintillator array of TA issensitive in effect only to the electromagnetic component of the air shower. By this reason, the energy
measurement of TA is less affected by the difference of the primary composition and the detail of unknown
hadronic interactions at EHE. This is the advantage with respect to the use of water Cherenkov counter,
which is equally sensitive to the high energy muons and soft gamma rays. We expect the uncertainty of theabsolute energy scale can be cut down to below 10% by carefully analyzing the simultaneously measured
events for E > 1019 eV. All together, an energy spectrum with twice smaller statistical error than the presentAGASA will be obtained by the 3-year measurement of the array and the telescope.
The angular resolution of the TA ground array will be about 10 and is better than 1.60 of AGASA by virtueof the full wave form sampling of the PMT signal. The angular resolution expected for the fluorescence andhybrid events are significantly better and they contribute for identifying the cluster events particularly at thelower energy below 1019.6 eV. In addition, the magnetic deflection by the galactic field is smaller and moreregular in the northern sky, which will be advantageous to confirm the cluster event of AGASA and to104

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search for corresponding astronomical sources. If the primary EHECRs are charged particles, systematic
magnetic deflections with respect to the direction of the galactic field would be observed, which may be
used for determining the chemical composition of the primary cosmic rays. Some of the EHECR source
models also predict a large scale anisotropy of arrival directions.
For identifying the origin of EHECRs, it is essential to identify the particle species of the primary cosmic ray.
For example, many of the particle physics oriented models predict an abundant generation of EHE gamma
rays and neutrinos rather than protons. On the other hand, the conventional shock wave acceleration will bestrongly supported if heavy nuclei are identified as the major composition of EHECRs. For the TA, theparticle identification is provided by the shower profile measurement by the fluorescence telescopes.
Photons with energies greater than 5 x 1018 eV may be identified by the Landau - Pomeranchuk - Migdal(LPM) effect leading to a deeply penetrating shower in the atmosphere. Above 2 x 1019 eV, a geomagnetic
cascade starts and the LPM effect is largely cancelled. The geomagnetic effect is strongly related with theCR arrival direction with respect to the Earth magnetic field and will clearly be identified by the north-south
asymmetry when enough samples are collected.
The EHE neutrinos produce nearly horizontal showers and are easy to identify. The target volume of the firstphase TA for EHE neutrino is ~1010 ton sr and the effective neutrino aperture is ~0.03 km2 sr for energies
above 1020 eV. The expected neutrino event rate is 0.04 - 2 event in 10 years for the decay of super-heavy
relic particles or the Z-burst model depending on the assumption of models. In general, the detection of EHE
gamma rays and neutrinos is hard with a limited acceptance of the first phase hybrid TA. We will have towait for a future deployment of the full TA or the AGASA x 100 large ground arrays.
As of June 2005, 18 scintillation counters and 2 telescopes are deployed in the field of Utah and are being
tested. The mass deployment will start later in 2005 and will be completed by March 2007. We expect thatastonishing discoveries of AGASA on EHECRs are confirmed or refuted by 2010.

4. Acknowledgements
The discussion with colleagues in AGASA, HiRes and TA collaborations is highly appreciated. The
Telescope Array is being constructed by the support of Grant-in-Aid for Scientific Research (Kakenhi) on
the Priority Area "The Highest Energy Cosmic Rays" by the Ministry of Education, Culture, Sports, Scienceand Technology of Japan.

References[1] K. Greisen, Phys. Rev. Lett. 16 (1966) 748.
T. Zatsepin and V. A. Kuzmin, JETP Lett. 4 (1966) 178.
[2] N.Chiba et al., Nucl. Instrum. Meth. A311 (1992) 338.
[3] M. Takeda et al., Phys. Rev. Lett. 81 (1998) 1163.
N. Hayashida et al., Astrophys. J. 522 (1999) 225.
M. Takeda et al., Astropart. Phys. 19 (2003) 447.
[4] M. Chikawa et al, Proceedings of the 27th ICRC (Hamburg 2001) HE1.4-3.
[5] Y. Uchihori et al., Astropart. Phys. 13 (2000) 151.
[6] For review of EHECR physics, see M. Nagano & A.A. Watson, Rev. Mod. Phys. 72 (2000) 689.
[7] T. Abu-Zayyad et al., Phys. Rev. Lett. 92 (2004) 151101.
T. Abu-Zayyad et al., Astropart. Phys. 23 (2005) 157.
[8] The Telescope Array Project: Design Report. July 2000.